Embedded Files
Introduction
While running commands (such as netstat) can provide useful information, sometimes you need to drill down more to the packet level. This is where tracing tools come in handy, such as iptrace, ipreport, and tcpdump. You'll also learn how you can tune a network using tools such as no. The no command is similar to vmo and ioo, but no is the network flavor. This article focuses on tcp workload tuning, udp workload tuning, and some other noteworthy parameters with the no utility. The article also addresses ARP cache tuning and how to monitor and tune ARP statistics. You will also look at name resolution and how you can easily increase performance by making small adjustments to resolve hostnames.
Monitoring network packets
In this section, you'll see an overview of tools available to help you monitor your network packets. These tools allow you to troubleshoot a performance problem quickly and capture data for historical trending and analysis.
Part 1 of this series addressed some of the very basic flags, such as -in, that you typically use with netstat. Using netstat, you can also monitor more detailed information about the packets themselves. For example, the -D option shows the overall number of packets received, transmitted, and dropped in your communication subsystem. The results are sorted by device, driver, and protocol (see Listing 1).
Listing 1. netstat with the -D option
l488pp065_pub[/tmp] > netstat -D
Source Ipkts Opkts Idrops Odrops
-------------------------------------------------------------------------------
ent_dev1 71306227 337207 0 0
ent_dev0 203313084 82292 0 0
---------------------------------------------------------------
Devices Total 274619311 419499 0 0
-------------------------------------------------------------------------------
ent_dd1 71306227 337207 0 0
ent_dd0 203313084 82292 0 0
---------------------------------------------------------------
Drivers Total 274619311 419499 0 0
-------------------------------------------------------------------------------
ent_dmx1 70327758 N/A 978469 N/A
ent_dmx0 202846759 N/A 466325 N/A
---------------------------------------------------------------
Demuxer Total 273174517 N/A 1444794 N/A
-------------------------------------------------------------------------------
IP 204276236 1063977 899839 828213
IPv6 70588 70588 0 6208
TCP 714368 785630 72 0
UDP 202697468 319900 202172157 0
---------------------------------------------------------------
Protocols Total 407688072 2169507 203072068 828213
-------------------------------------------------------------------------------
en_if1 70327759 337207 0 0
en_if0 202846780 82315 0 0
lo_if0 780891 780890 12 0
---------------------------------------------------------------
Net IF Total 273955430 1200412 12 0
-------------------------------------------------------------------------------
NFS/RPC Client 24 N/A 0 N/A
NFS/RPC Server 0 N/A 0 N/A
NFS Client 279 N/A 0 N/A
NFS Server 0 N/A 0 N/A
---------------------------------------------------------------
NFS/RPC Total N/A 303 0 0
-------------------------------------------------------------------------------
(Note: N/A -> Not Applicable)
Another useful flag is the -s, which shows detailed statistics for all protocols used, including packets sent, received and dropped. If you only want to view tcp, you can also use the -p flag (see Listing 2).
Listing 2. netstat with the -p option
l488pp065_pub[/tmp] > netstat -p tcptcp:
785684 packets sent
227657 data packets (13800898 bytes)
0 data packets (0 bytes) retransmitted
279285 ack-only packets (509 delayed)
0 URG only packets
0 window probe packets
69732 window update packets
418020 control packets
0 large sends
0 bytes sent using largesend
0 bytes is the biggest largesend
714418 packets received
360033 acks (for 14009902 bytes)
69728 duplicate acks
0 acks for unsent data
217241 packets (1660096 bytes) received in-sequence
72 completely duplicate packets (72 bytes)
0 old duplicate packets
0 packets with some dup. data (0 bytes duped)
69632 out-of-order packets (0 bytes)
0 packets (0 bytes) of data after window
0 window probes
69733 window update packets
0 packets received after close
0 packets with bad hardware assisted checksum
0 discarded for bad checksums
0 discarded for bad header offset fields
0 discarded because packet too short
0 discarded by listeners
0 discarded due to listener's queue full
5241 ack packet headers correctly predicted
75327 data packet headers correctly predicted
69655 connection requests
69712 connection accepts
139364 connections established (including accepts)
139417 connections closed (including 12 drops)
0 connections with ECN capability
0 times responded to ECN
2 embryonic connections dropped
429685 segments updated rtt (of 429463 attempts)
0 segments with congestion window reduced bit set
0 segments with congestion experienced bit set
0 resends due to path MTU discovery
1 path MTU discovery termination due to retransmits
3 retransmit timeouts
0 connections dropped by rexmit timeout
0 fast retransmits
0 when congestion window less than 4 segments
0 newreno retransmits
0 times avoided false fast retransmits
0 persist timeouts
0 connections dropped due to persist timeout
0 keepalive timeouts
0 keepalive probes sent
0 connections dropped by keepalive
0 times SACK blocks array is extended
0 times SACK holes array is extended
0 packets dropped due to memory allocation failure
0 connections in timewait reused
0 delayed ACKs for SYN
0 delayed ACKs for FIN
0 send_and_disconnects
0 spliced connections
0 spliced connections closed
0 spliced connections reset
0 spliced connections timeout
0 spliced connections persist timeout
0 spliced connections keepalive timeout
0 TCP checksum offload disabled during retransmit
0 Connections dropped due to bad ACKs
There are actually so many different ways of using netstat that the best place to start is by looking at the man page and go from there. Don't be afraid to run these commands, because they won't eat up disk space or affect performance. The tracing tools that are provided within AIX 7 are used to record detailed information about the packets. Use them with more caution. These tools are extremely helpful when trying to determine the root cause of network performance problems.
First, look at iptrace and ipreport. The iptrace command records all the packets received from the network interfaces. Theipreport command formats the data that is generated from iptrace into a readable trace report. Further, you can also useipfilter to sort the output file created from ipreport. Try starting the trace and keep it going for one minute (see Listing 3).
Listing 3. Starting the trace
l488pp065_pub[/tmp] > /usr/sbin/iptrace -a -i en0 iptrace.out &
[1] 12845206
l488pp065_pub[/tmp] > [8126632]
[1] + Done /usr/sbin/iptrace -a -i en0 iptrace.out &
l488pp065_pub[/tmp] > ps -ef | grep iptrace
root 8126632 1 15 05:54:55 - 0:00 /usr/sbin/iptrace
-a -i en0 iptrace.out
root 14221424 7012524 7 05:55:17 pts/1 0:00 grep iptrace
When you are done with the trace, you need to kill the process (see Listing 4).
Listing 4. Killing the process
l488pp065_pub[/tmp] > kill -1 8126632l488pp065_pub[/tmp]
> iptrace: unload success!
l488pp065_pub[/tmp] > ipreport -r -s iptrace.out >/ipreport.network
Now, examine the output (see Listing 5):
Listing 5. Examining the output
l488pp065_pub[/tmp] > more /ipreport.network
IPTRACE version: 2.0
ETH: ====( 114 bytes transmitted on interface en0 )==== 05:54:55.151599119
ETH: [ 66:da:93:d1:6b:17 -> 6e:87:70:00:40:03 ] type 800 (IP)
IP: < SRC = 172.29.148.225 > (l488pp065_pub)
IP: < DST = 172.29.131.16 >
IP: ip_v=4, ip_hl=20, ip_tos=16, ip_len=100, ip_id=49399, ip_off=0 DF
IP: ip_ttl=60, ip_sum=d60, ip_p = 6 (TCP)
TCP: <source port=22(ssh), destination port=54678 >
TCP: th_seq=47587592, th_ack=3002348404
TCP: th_off=8, flags<PUSH | ACK>
TCP: th_win=65522, th_sum=0, th_urp=0
TCP: nop
TCP: nop
TCP: timestamps TSVal: 0x4f486827 TSEcho: 0x4c8da569
TCP: 00000000 9fec0a46 c8dd1c9b 98ff0213 87c714c0 |...F............|
TCP: 00000010 0ec081aa 7c76335f 0bfd0d8f 63d0bf1a |....|v3_....c...|
TCP: 00000020 808359b4 13e1a29d 4dacdd51 dad01053 |..Y.....M..Q...S|
Listing 5 shows the captured information about each packet, including packet size and IP address information. As you can imagine, the trace file can get very large quickly. The example file grew to 40MB in less than one minute! Be very careful when running these traces, because you will run out of disk space really fast if you don't have the disk bandwidth for these files.
You can also start the trace using the System Resource Controller (SRC). See Listing 6.
Listing 6. Starting the trace using SRC
l488pp065_pub[/tmp] > startsrc -s iptrace -a "-i
en1 /home/testing/iptrace/iptracelog"0513-059 The iptrace Subsystem
has been started. Subsystem PID is 12845270.
l488pp065_pub[/tmp] > stopsrc -s iptrace
0513-004 The Subsystem or Group, iptrace, is currently inoperative.
What about tcpdump? tcpdump prints out headers of the packets, which are captured for each NIC. One important difference with tcpdump is that, unlike iptrace, it can look at only one network interface at a time. And, because iptrace examines the entire packet from the kernel space, the results can offer lots of dropped packets. With tcpdump, you can also limit the amount of data to be traced. Also, you do not need to use an ipreport type of command to format binary data, because tcpdump does the trace and the output. See Listing 7 for an example.
Listing 7. Using tcpdump
l488pp065_pub[/tmp] > tcpdump > tcp.outtcpdump: listening on
en0, link-type 1, capture size 96 bytes
tcpdump continues to capture packets until you hit Ctrl+C. If any packets were dropped due to a lack of buffer space, it reports that, too.
Listing 8 shows what you see when you end the example trace and view the file.
Listing 8. End of the trace
28 packets received by filter0 packets dropped by kernel
l488pp065_pub[/tmp] > cat tcp.out
06:00:21.003328 IP l488pp065_pub.ssh > 172.29.131.16.54678:
P 47609416:47609464(48) ack 3002357700 win 65522 <nop,nop,timestamp 133014597
1 1284351989>
06:00:21.003387 IP l488pp065_pub.ssh > 172.29.131.16.54678: P 48:208(160)
ack 1 win 65522 <nop,nop,timestamp 1330145971 1284351989>
06:00:21.028081 IP 172.29.131.16.54678 > l488pp065_pub.ssh: . ack 208 win
32761 <nop,nop,timestamp 1284351989 1330145971>
06:00:21.238937 ARP, Request who-has 172.29.173.186 tell 172.29.133.221, length 46
06:00:21.239110 ARP, Request who-has 172.29.173.236 tell 172.29.129.59, length 46
06:00:21.325060 ARP, Request who-has 172.29.175.252 tell 172.29.129.58, length 46
06:00:21.464383 IP6 fe80::4464:ceff:fe65:4f0c > ff02::1:ff41:34d0: ICMP6,
neighbor solicitation, who has fe80::221:5eff:fe41:34d0, length
32
06:00:21.505281 ARP, Request who-has 172.29.175.60 tell 172.29.133.223, length 46
06:00:22.013530 ARP, Request who-has 172.29.174.66 tell 172.29.133.222, length 46
06:00:22.054164 ARP, Request who-has 172.29.173.237 tell 172.29.129.59, length 46
06:00:22.076819 ARP, Request who-has 172.29.122.25 tell 172.29.122.2, length 46
06:00:22.393898 IP 172.29.148.116.32852 > 239.255.255.253.svrloc: UDP, length 56
06:00:22.464355 IP6 fe80::4464:ceff:fe65:4f0c > ff02::1:ff41:34d0: ICMP6, neighbor
solicitation, who has fe80::221:5eff:fe41:34d0, length
32
06:00:22.935140 802.1d config 8000.00:16:60:f9:a8:00.8011 root 8000.00:16:60:f9:a8:00
pathcost 0 age 0 max 20 hello 2 fdelay 15
06:00:23.186380 ARP, Request who-has 172.29.122.26 tell 172.29.122.2, length 46
06:00:24.520770 ARP, Request who-has 172.29.175.60 tell 172.29.133.223, length 46
06:00:24.558139 ARP, Request who-has 172.29.175.252 tell 172.29.129.58, length 46
06:00:24.573524 ARP, Request who-has 172.29.175.5 tell 172.29.129.57, length 46
06:00:24.736838 IP 172.29.148.116.32853 > 239.255.255.253.svrloc: UDP, length 56
06:00:24.931436 802.1d config 8000.00:16:60:f9:a8:00.8011 root 8000.00:16:60:f9:a8:00
pathcost 0 age 0 max 20 hello 2 fdelay 15
06:00:25.029112 IP 172.29.133.222.netbios-dgm >
172.29.191.255.netbios-dgm: UDP, length 201
06:00:25.029965 IP 172.29.133.222.netbios-dgm >
172.29.191.255.netbios-dgm: UDP, length 201
06:00:25.030751 IP 172.29.133.222.netbios-dgm >
172.29.191.255.netbios-dgm: UDP, length 201
06:00:25.031674 IP 172.29.133.222.netbios-dgm >
172.29.191.255.netbios-dgm: UDP, length 201
06:00:25.032636 IP 172.29.133.222.netbios-dgm >
172.29.191.255.netbios-dgm: UDP, length 201
06:00:25.033647 IP 172.29.133.222.netbios-dgm >
172.29.191.255.netbios-dgm: UDP, length 201
06:00:25.033732 CDP v2, ttl: 180s, Device-ID 'Switch'[|cdp]
06:00:25.034738 IP 172.29.133.222.netbios-dgm >
172.29.191.255.netbios-dgm: UDP, length 201
06:00:25.035741 IP 172.29.133.222.netbios-dgm >
172.29.191.255.netbios-dgm: UDP, length 201
The main benefit of tcpdump is that you can specify filters so that you can select only particular protocols, sources, destinations, ports and other combinations. This is useful if you want diagnose or determine the NFS traffic, for example, between two hosts.
Tuning network performance
In this section, you will learn how to use the no command to tune your network subsystem. You'll also look at other areas that can impact network performance, and you'll learn about recommended tuning methodologies where appropriate.
The most important command for tuning network parameters is the no command. First, take a look at all the parameters, using the -a flag (see Listing 9). Be warned, though, that the list is quite extensive.
Listing 9. Viewing the parameters
l488pp065_pub[/tmp] > no -a
arpqsize = 12
arpt_killc = 20
arptab_bsiz = 7
arptab_nb = 149
bcastping = 0
clean_partial_conns = 0
delayack = 0
delayackports = {}
dgd_packets_lost = 3
dgd_ping_time = 5
dgd_retry_time = 5
directed_broadcast = 0
fasttimo = 200
icmp6_errmsg_rate = 10
icmpaddressmask = 0
ie5_old_multicast_mapping = 0
ifsize = 256
ip6_defttl = 64
ip6_prune = 1
ip6forwarding = 0
ip6srcrouteforward = 1
ip_ifdelete_notify = 0
ip_nfrag = 200
ipforwarding = 0
ipfragttl = 2
ipignoreredirects = 0
ipqmaxlen = 100
ipsendredirects = 1
ipsrcrouteforward = 1
ipsrcrouterecv = 0
ipsrcroutesend = 1
llsleep_timeout = 3
lo_perf = 1
lowthresh = 90
main_if6 = 0
main_site6 = 0
maxnip6q = 20
maxttl = 255
medthresh = 95
mpr_policy = 1
multi_homed = 1
nbc_limit = 262144
nbc_max_cache = 131072
nbc_min_cache = 1
nbc_ofile_hashsz = 12841
nbc_pseg = 0
nbc_pseg_limit = 524288
ndd_event_name = {all}
ndd_event_tracing = 0
ndp_mmaxtries = 3
ndp_umaxtries = 3
ndpqsize = 50
ndpt_down = 3
ndpt_keep = 120
ndpt_probe = 5
ndpt_reachable = 30
ndpt_retrans = 1
net_buf_size = {all}
net_buf_type = {all}
net_malloc_frag_mask = {0}
netm_page_promote = 1
nonlocsrcroute = 0
nstrpush = 8
passive_dgd = 0
pmtu_default_age = 10
pmtu_expire = 10
pmtu_rediscover_interval = 30
poolbuckets = 4
psebufcalls = 20
Alternatively, you can also use the -L flag. It provides much more detailed information, including current, default, boot, and range of settings. This can make it much easier to determine whether a given value is working at its optimum value and whether a specific item could be improved beyond it's current value. Listing 10 shows you only the first few lines.
Listing 10. Using the -L flag
l488pp065_pub[/tmp] > no -L
General Network Parameters
--------------------------------------------------------------------------------
NAME CUR DEF BOOT MIN MAX UNIT TYPE
DEPENDENCIES
--------------------------------------------------------------------------------
fasttimo 200 200 200 50 200 millisecond D
--------------------------------------------------------------------------------
nbc_limit 256K 256K 256K 0 8E-1 kbyte D
thewall
--------------------------------------------------------------------------------
nbc_max_cache 128K 128K 128K 1 256M byte D
nbc_min_cache
nbc_limit
--------------------------------------------------------------------------------
nbc_min_cache 1 1 1 1 128K byte D
nbc_max_cache
--------------------------------------------------------------------------------
nbc_ofile_hashsz 12841 12841 12841 1 999999 segment D
--------------------------------------------------------------------------------
nbc_pseg 0 0 0 0 2G-1 segment D
--------------------------------------------------------------------------------
nbc_pseg_limit 512K 512K 512K 0 1M kbyte D
--------------------------------------------------------------------------------
ndd_event_name {all} {all} {all} 0 128 string D
... trimmed for clarity
There are many parameters here. The thewall defines the upper limit for network kernel buffers. Today, its size is defined at installation time, depending on the amount of RAM and the kernel type. For example, if you are running AIX 5.3 on a 64-bit kernel, the parameter is set at half the size of real memory.
Listing 11. Setting the size of the thewall parameter
l488pp065_pub[/tmp] > no -a|grep thewall
thewall = 1048576
Part 1 discussed mbufs, but it's worth another mention here, because it relates to thewall. Remember that mbufs are used to store data in the kernel for both incoming and outgoing traffic. This is why determining the right amount of mbufs is extremely important. The value of the maxmbuf tunable limits the amount of memory that the communication systems use. If the value is 0,thewall tunable is used, and it cannot be modified from its default. Changing this tunable is a way to lower the thewall limit. As the default, if maxmbuf is 0, this value is used regardless of what thewall uses. netstat -m is used to detect shortages of failures of network memory requests (see Listing 12).
Listing 12. netstat with the -m option
l488pp065_pub[/tmp] > netstat -m
Kernel malloc statistics:
******* CPU 0 *******
By size inuse calls failed delayed free hiwat freed
64 558 2087455 0 7 274 5240 0
128 5884 1901723 0 175 164 2620 0
256 5780 653578 0 295 2876 5240 500
512 7970 182051630 0 972 102 6550 0
1024 3159 1960612 0 794 49 2620 0
2048 1069 3462138 0 520 25 3930 0
4096 2056 2794 0 83 3 1310 0
8192 5 260 0 3 163 327 0
16384 256 413 0 62 0 163 0
32768 55 274 0 23 4 81 0
65536 117 175 0 76 0 81 0
131072 4 5 0 0 102 204 0
... other CPU stats trimmed
Streams mblk statistic failures:
0 high priority mblk failures
0 medium priority mblk failures
0 low priority mblk failures
In the example, there are no shortages (failures).
Although there are many parameters you can change with the no utility, most of them are better left alone. The most important parameters are ones that refer to TCP streaming workload tuning.
tcp_sendspace —This controls how much buffer space in the kernel is used to buffer application data. You really want to bump this up from the default, because if its limit is reached, the sending application suspends data transfer until TCP sends the data to the buffer.
tcp_receivespace —In addition to controlling the amount of buffer space to be consumed by receive buffers, AIX 7 also uses this value to determine the size to make its transmit window.
udp_sendspace —With UDP, you can set this to no more than 65536, because IP has an upper limit of 65536 bytes per packet.
udp_resvspace —This value should be greater than udp_sendpsace, because it needs to handle as many simultaneous UDP packets per socket as it can. This parameter can easily be set to 10 times the value ofudp_sendspace.
Now, let's make some changes. First, increase the size of udp_sendspace (see Listing 13).
Listing 13. Increasing the size of udp_sendspace
l488pp065_pub[/tmp] > no -p -o udp_sendspace=65536Setting udp_sendspace to 65536
Setting udp_sendspace to 65536 in nextboot file
Change to tunable udp_sendspace, will only be effective for future connections
Next, change udp_recsvspace to the recommended configuration of 10 times udp_sendspace). See Listing 14.
Listing 14. Changing udp_recsvspace
l488pp065_pub[/tmp] > no -p -o udp_recvspace=655360Setting udp_recvspace to 655360
Setting udp_recvspace to 655360 in nextboot file
Change to tunable udp_recvspace, will only be effective for future connections
Note that the -p flag keeps the entries, even after a reboot. It appends the /etc/tunables/nextboot stanza file, as shown in Listing 15.
Listing 15. Looking at the /etc/tunables/nextbook file
no:
udp_recvspace = "655360"
udp_sendspace = "65536"
Regarding the tcp parameters for higher speed adapters, there is no problem setting tcp_sendspace to twice the value oftcp_recvspace. For example, you can use the settings in Listing 16.
Listing 16. Examples settings for tcp_sendspace
tcp_receivespace = 262144
tcp_sendspace= 524288
Other important workload parameters include rfc1323 and sb_max.
The rfc1323 tunable enables the TCP window scaling option, which allows TCP to use a larger window size. Turning it on enables the best TCP performance. The sb_max tunable sets an upper limit on the number of socket buffers queued to an individual socket, which controls the amount of buffer space consumed by buffers (queued to either a sender or received socket). This amount should usually be less than the wall and approximately 4 times the size of the largest value of the tcp or udp send and receive settings. For example, if your udp_recvspace is 655360, you can't go wrong if you double it to 1310720.
Now look at tcp_nodelayack. This tunable prompts TCP to send an immediate acknowledgement, rather than a delayed acknowledgement. While this can add more overhead in some environments, it can greatly improve network performance in others. If you change this parameter, but it does not improve performance, you can quickly change it back.
Next look at ipqmalen. This tunable controls the length of the IP input queue. If you see an overflow counter (through the use ofnetstat -s), setting a maximum length of this queue can help fix the overflow.
What about ARP? When many clients are connected to the system, you might want to tune the ARP cache. You can look at the statistics using netstat (see Listing 17).
Listing 17. Using netstat with -p arp
l488pp065_pub[/tmp] > netstat -p arparp:
12 packets sent
0 packets purged
If you see a high purge count, increase the size of the ARP table. For the example table, this isn't needed.
Here are the no parameters that relate to ARP (see Listing 18).
Listing 18. Using the no parameters
l488pp065_pub[/tmp] > no -a | grep arp
arpqsize = 12
arpt_killc = 20
arptab_bsiz = 7
arptab_nb = 149
You can view the specific interface settings using either ifconfig or lsattr. In the example in Listing 19, look at the settings using ifconfig (look at the last line which references some of the tunables mentioned earlier).
Listing 19. Viewing specific interface settings using ifconfig
l488pp065_pub[/tmp] > ifconfig en0en0: flags=1e080863,480<P,BROADCAST,NOTRAILERS,
RUNNING,SIMPLEX,MULTICAST,GROUPRT,64BIT,CHECKSUM_OFFLOAD(ACTIVE),CHAIN>
inet 172.29.148.225 netmask 0xffffc000 broadcast 172.29.191.255
tcp_sendspace 262144 tcp_recvspace 262144 rfc1323 1
You can change these options (by interface) by using SMIT, chdev, or ifconfig. Note that ifconfig will not update the Object Data Manager (ODM). Therefore, on a reboot, it will revert to the previous value. Because of that, you should use SMIT: the fastpath of smit tcpip>further configuration>Network interfaces>Change/Show characteristics of an interface (see Figure 1).
Figure 1. Using SMIT to change interface settings
You might wonder why the no parameters don't apply to some interfaces. Name resolution is another area that can impact performance. If you know how you want to resolve (using DNS or the hosts file), make sure name resolution is set up correctly in the /etc/netsvc.conf file. Look at a piece of the file in Listing 20.
Listing 20. Piece of /etc/netsvc.conf file
# Example:
# aliases = nis, files
#
hosts=local,bind4
If you're using DNS, take out the local if you are not using a host's file at all, or you can leave it in if you are using it as a backup to DNS (but make it the second entry). Alternatively, take out the bind if you're not using DNS at all, because it will only slow down your performance by first attempting (if it is the first entry in the record) to resolve using a Name Server that doesn't exist.
Summary
This article discussed how to monitor network packets on the network. You used netstat and drilled down to the packet level using tracing tools, such as iptrace and tcpdump. Further, you learned how to tune your network using the no utility. Using this utility, you explored tcp and udp workload tuning while also learning some other noteworthy parameters. You made tuning changes and read about how you might want to tune certain settings. You also examined ARP cache tuning and saw how you could monitor and tune ARP statistics. You looked at ISNO and learned how you could tune specific no tunables by interface. You also looked at name resolution and how you could easily increase performance by making small adjustments in how to resolve hostnames.
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